Cordless telephone set

ABSTRACT

Disclosed is a cordless telephone set capable of suppressing power consumption during power failures to lengthen the time for a call. A cordless telephone ( 1 ) receives power supply from a handset ( 3 ) placed on a charging section ( 26 ) of a base unit ( 2 ), during power failures. In order to suppress current consumption of a secondary battery (BT) of the handset ( 3 ), the base unit ( 2 ) instructs a base unit radio section ( 21 ) to be switched to a transmission power-saving mode in which transmission power of the base unit radio section ( 21 ) is reduced down to a level at which no communication error occurs, according to radio wave information transmitted from the handset ( 3 ) and indicates the received signal strength of a radio signal from the base unit radio section ( 21 ) of the base unit ( 2 ), received by a handset radio section ( 39 ) of the handset ( 3 ).

TECHNICAL FIELD

The present invention relates to a cordless telephone set which includesa cordless handset which is supplied with electric power from asecondary battery and a base unit which is supplied with electric powerfrom a power line.

BACKGROUND ART

A cordless telephone set has high convenience since a user can make acall to the other party through a cordless handset while being at adistance from a base unit connected to a telephone line. However, duringa power failure, the base unit cannot obtain electric power from a powerline and cannot communicate with the cordless handset. Thus, the usercannot make a call to the other party.

In this respect, a cordless telephone set has been proposed with which auser can make a call to the other party even during a power failure. Forexample, Patent Literature 1 discloses a cordless telephone set in whichif it is detected that a radio link between a cordless handset and abase unit is cut off, a switch circuit of the cordless handset isswitched, and thus, electric current from a battery flows in the baseunit as backup current so that the base unit and the cordless handsetcan communicate with each other even during a power failure.

CITATION LIST Patent Literature

Patent Literature 1.: JP2007-243281A

SUMMARY OF INVENTION Technical Problem

However, the cordless telephone set disclosed in Patent Literature 1 hasa problem that the weight of the cordless handset becomes heavy sincecircuit elements such as a switch circuit and a detection unit areinstalled in the cordless handset, differently from a normal cordlesstelephone set which does not support the power-failure feature.

Accordingly, an object of the invention is to provide a cordlesstelephone set in which electric power can be supplied to a base unitfrom a secondary battery of a handset in a case where voltage from anexternal power supply is reduced due to a power failure, plug-out or thelike and a light-weight cordless handset can be achieved.

Solution to Problem

An aspect of the invention provides a cordless telephone set whichperforms radio communication between a handset and a base unit, whereinthe base unit includes a first radio communicating section whichperforms modulation and demodulation with voltage from an external powersupply, and a charging circuit which outputs a predetermined voltagebased on the voltage from the external power supply, wherein the handsetincludes a secondary battery which is charged by the predeterminedvoltage output from the secondary battery when the handset is placed onthe base unit, and a second radio communicating section which performsmodulation and demodulation with voltage output from the secondarybattery, and wherein the base unit further includes a determination unitwhich determines whether the voltage from the external power supplyexceeds a predetermined value, and a switch circuit which cuts off powersupply to the first radio communicating section from the secondarybattery of the handset placed on the base unit when the determinationunit has determined that the voltage from the external power supplyexceeds the predetermined value and allows power supply to the firstradio communicating section from the secondary battery of the handsetplaced on the base unit when the determination unit has determined thatthe voltage from the external power supply does not exceed thepredetermined value.

Advantageous Effects of Invention

In the cordless telephone set according to the aspect of the invention,since the determination unit which determines that the voltage from theexternal power supply is reduced and the switch circuit which allowspower supply to the base unit from the secondary battery built-in in thehandset are not installed on the side of the handset, it is possible toachieve a light-weight handset and to continue a voice call withoutimposing a burden on the user.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view illustrating a cordless telephone setaccording to an embodiment of the invention.

FIG. 2 is a block diagram illustrating a configuration of the cordlesstelephone set in FIG. 1.

In FIG. 3, (A) is a plan view illustrating a base unit of the cordlesstelephone set shown in FIG. 1, and (B) is a diagram of the base unit ofthe cordless telephone set shown in FIG. 1, when seen from a rear side.

FIG. 4 is a perspective view illustrating a handset of the cordlesstelephone set shown in FIG. 1.

FIG. 5 is a circuit diagram illustrating an example of a voltagedetection circuit of the base unit shown in FIG. 2.

FIG. 6 is a circuit diagram illustrating an example of a base unitcharging circuit of the base unit shown in FIG. 2.

FIG. 7 is a circuit diagram illustrating an example of a switch circuitof the base unit shown in FIG. 2.

FIG. 8 is a circuit diagram illustrating an example of a handsetcharging circuit of the handset shown in FIG. 2.

FIG. 9 is a circuit diagram illustrating an example of a handsetconstant current circuit of the handset shown in FIG. 2.

FIG. 10 is a flowchart illustrating an operation of the base unit of thecordless telephone set shown in FIG. 1.

FIG. 11 is a flowchart illustrating an operation of the handset of thecordless telephone shown in FIG. 1 which is placed on a charging sectionof the base unit.

FIG. 12 is a flowchart illustrating, when a handset is placed on acharging section of a base unit, an operation of a different handset.

FIG. 13 is a flowchart illustrating, when a handset is not placed on acharging section of a base unit, an operation of a different handset.

FIG. 14 is a flowchart illustrating an operation of a handset,subsequent to FIG. 13.

FIG. 15 is a diagram illustrating a transmission power adjustmentperformed by a base unit.

FIG. 16 is a characteristic diagram illustrating the relationshipbetween a reception level, transmission power of a base unit and acommunication error rate in a handset.

FIG. 17 is a diagram illustrating an example of a state where a messageis displayed on a handset display section of the cordless telephone setaccording to the embodiment of the invention.

FIG. 18 is a diagram illustrating an example of a state where a messageis displayed on the handset display section of the cordless telephoneset according to the embodiment of the invention.

FIG. 19 is a diagram illustrating an example of a state where a messageis displayed on the handset display section of the cordless telephoneset according to the embodiment of the invention.

FIG. 20 is a diagram illustrating an example of a state where a messageis displayed on the handset display section of the cordless telephoneset according to the embodiment of the invention.

FIG. 21 is a diagram illustrating an example of a state where a messageis displayed on the handset display section of the cordless telephoneset according to the embodiment of the invention.

FIG. 22 is a diagram illustrating an example of a state where a messageis displayed on the handset display section of the cordless telephoneset according to the embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

According to a first aspect of the invention, there is provided acordless telephone set which performs radio communication between ahandset and a base unit, wherein the base unit includes a first radiocommunicating section which performs modulation and demodulation withvoltage from an external power supply, and a charging circuit whichoutputs a predetermined voltage based on the voltage from the externalpower supply, wherein the handset includes a secondary battery which ischarged by the predetermined voltage output from the secondary batterywhen the handset is placed on the base unit, and a second radiocommunicating section which performs modulation and demodulation withvoltage output from the secondary battery, and wherein the base unitfurther includes a determination unit which determines whether thevoltage from the external power supply exceeds a predetermined value,and a switch circuit which cuts off power supply to the first radiocommunicating section from the secondary battery of the handset placedon the base unit when the determination unit has determined that thevoltage from the external power supply exceeds the predetermined valueand allows power supply to the first radio communicating section fromthe secondary battery of the handset placed on the base unit when thedetermination unit has determined that the voltage from the externalpower supply does not exceed the predetermined value.

According to the first aspect, since the determination unit whichdetermines that the voltage from the external power supply is reducedand the switch circuit which allows power supply to the base unit fromthe secondary battery built-in in the handset are not installed on theside of the handset, it is possible to achieve a light-weight handsetand to continue a voice call without imposing a burden on the user.

According to a second aspect of the invention, in the cordless telephoneset in the first aspect, the base unit further includes a first controlsection which controls transmission power of the first radiocommunicating section, the first radio communicating section receives,from the handset, radio wave information which indicates a receivedsignal strength of a radio signal from the first radio communicatingsection which is received by the second radio communicating section, andthe first control section reduces the transmission power of the firstradio communicating section in response to the received signal strengthindicated by the radio wave information when the determination unit hasdetermined that the voltage from the external power supply does notexceed the predetermined value.

According to the second aspect, when the voltage from the external powersupply is reduced, the cordless telephone set is switched to atransmission power-saving mode, and it is thus possible to continue avoice call over a long time.

According to a third aspect of the invention, in the cordless telephoneset in the second aspect, the first control section reduces thetransmission power of the first radio communicating section down to alevel at which no communication error occurs.

According to the third aspect, since the transmission power is reduceddown to the level at which no communication error occurs, it is possibleto efficiently consume the secondary battery, and to continue a voicecall over a long time.

According to a fourth aspect of the invention, in the cordless telephoneset in the first aspect, the handset further includes a second controlsection which starts up a hand-free call function when the handset isplaced on the base unit.

According to the fourth aspect, when there is an incoming call in astate where the handset is placed on the base unit, a hand-free callautomatically starts without a particular operation of a user forstarting up the hand-free call function, and thus, it is possible tomake a call in a state where the cordless handset is placed on the baseunit.

According to a fifth aspect of the invention, the handset furtherincludes a display section which displays predetermined data, the firstradio communicating section transmits predetermined information to thehandset when the determination unit has determined that the voltage fromthe external power supply does not exceed the predetermined value, andthe display section displays a state where power supply to the base unitfrom the external power supply is cut off when the second radiocommunicating section receives the predetermined information.

According to the fifth aspect, in the cordless telephone set in thefirst aspect, when the voltage from the external power supply isreduced, it is possible to indicate abnormality of power to the user ofthe handset, or to encourage the user to place the handset on a chargingsection of the base unit.

According to a sixth aspect of the invention, in the cordless telephoneset in the first aspect, the base unit further includes a capacitorwhich receives electricity with the voltage from the external powersupply or the voltage from the secondary battery.

According to the sixth aspect, even if the external power supply to thebase unit is stopped in a state where the cordless handset is separatedfrom the base unit, or even if the base unit is operated with thesecondary battery in the cordless handset in a state where the externalpower supply is stopped and the cordless handset is removed from thebase unit in this state, the operation of the base unit is maintained byelectric power from the capacitor of a large capacity for a moment, andit is thus possible to maintain a call in a state where the cordlesshandset and the base unit are separated from each other without suddendisconnection of the call.

According to a seventh aspect of the invention, in the cordlesstelephone set in the first aspect, the cordless telephone set includes aplurality of handsets, and when any one of the plurality of handsets isplaced on the base unit, the first radio communicating sectioncommunicates with a different handset which is not placed on the baseunit with a predetermined voltage output from the secondary battery.

According to the seventh aspect, when the voltage from the externalpower supply is reduced, if one of the plurality of handsets which isable to communicate with the base unit is placed on the base unit, it ispreferable that the secondary battery of this handset mainly supplyelectric power for operating the base unit, and it is not necessary thatthis cordless handset perform a transmission operation for a call,thereby making it possible to suppress power consumption. Further, it ispossible for a user to use the other cordless handsets in the state ofbeing separated from the base unit for a call, thereby securingconvenience as the cordless telephone set.

According to an eighth aspect of the invention, in the cordlesstelephone set in the first aspect, the switch circuit includes asemiconductor device including a function as a switching element such asa transistor or an electric field effect transistor.

According to the eighth aspect, since the semiconductor device is usedas the switch circuit, it is possible to enhance reliability of thecordless telephone set.

According to a ninth aspect of the invention, in the cordless telephoneset in the first aspect, the handset further includes a display sectionwhich displays predetermined data, the first radio communicating sectiontransmits predetermined information to the handset when thedetermination unit has determined that the voltage from the externalpower supply does not exceed the predetermined value, and the displaysection performs a display for preventing the handset placed on the baseunit from being removed from the base unit when the display section hasreceived the predetermined information by the second radio communicatingsection and the switch circuit allows power supply to the first radiocommunicating section from the secondary battery.

According to the ninth aspect, it is possible to prevent a user fromremoving the handset from the base unit by mistake during operationthrough the secondary battery of the handset.

According to a tenth aspect of the invention, in the cordless telephoneset in the first aspect, the handset further includes a display sectionwhich displays predetermined data, the first radio communicating sectiontransmits predetermined information to the handset when thedetermination unit has determined that the voltage from the externalpower supply does not exceed the predetermined value, and the displaysection performs a display for encouraging a user to place the handseton the base unit when the display section has received the predeterminedinformation by the second radio communicating section.

According to the tenth aspect, even though the user does not know thatthe voltage from the external power supply is reduced, it is possible toencourage the user to place the handset on the base unit.

Embodiment

A cordless telephone set according to an embodiment of the inventionwill be described with reference to the accompanying drawings.

A cordless telephone set 1 shown in FIG. 1 includes a base unit 2 whichis connected to a telephone line, two cordless handsets 3 (3 x and 3 y)which perform a radio communication with the base unit 2. Hereinafter, acordless handset is simply referred to as a handset. In FIG. 1, onecordless handset 3 x is placed on a charging section of the base unit 2and the other cordless handset 3 y is placed on a different charger 4.

The base unit 2 is supplied with electric power by an AC adaptor 2 awhich is connected to a power line through a socket. The handset 3 x issupplied with electric power by being placed on the base unit 2, inwhich a secondary battery which is installed in the handset 3 x ischarged. The handset 3 y is supplied with electric power from an ACadaptor 4a which is connected to a power line while being placed on thecharger 4, in which a secondary battery which is installed in thehandset 3 y is charged.

As shown in FIG. 2, the base unit 2 includes a telephone section 20which is an example of a first control section, a base unit radiosection 21 which is an example of a first radio communication section, apower circuit 22, a base unit constant voltage circuit 23, a voltagedetection circuit 24, a base unit charging circuit 25, a chargingsection 26, a switch circuit 27, a capacitor 28, and a speaker section29.

The telephone section 20 includes a line interface section 20 a and abase unit control section 20 b. The telephone section 20 may be realizedas an integrated circuit. Similarly, the base unit radio section 21 maybe also realized as an integrated circuit. Further, both functions ofthe telephone section 20 and the base unit radio section 21 may becombined as a single integrated circuit.

The line interface section 20 a is connected to a telephone line,detects an incoming signal, and has a function of mutually converting anaudio analog signal through the telephone line and an analog digitalsignal with respect to the base unit control section 20 b. As shown in(B) of FIG. 3, the line interface 20 a is connected to the telephoneline in a state where an end of a telephone cable is connected to adial-up connector 2 c which has a socket installed downward a recesssection 2 b on a rear side and the other end is connected to a modularjack installed on a wall surface.

The base unit control section 20 b shown in FIG. 2 overall controls theentire base unit 2. The base unit control section 20 b transmits andreceives an audio digital signal to and from the handset 3 through thebase unit radio section 21, transmits power failure information to thehandset 3, or adjusts transmission power with respect to the base unitradio section 21. The base unit control section 20 b includes aprocessor and a peripheral circuit for operating the processor.

The base unit radio section 21 performs modulation and demodulation sothat the base unit 2 which is a main control station and the handset 3which is a slave station can communicate with each other through TDMA(time-division multiple access), and transmits and receives an audiodigital signal to and from the handset 3 through an antenna 21 a.Further, the base unit radio section 21 includes a function of adjustingtransmission power of a radio signal to the handset 3 by an instructionfrom the base unit control section 20 b.

The power circuit 22 is a DC/DC converter which reduces a DC voltagefrom the AC adaptor 2 a which is connected to an external powerconnector 2 d installed in the recess section 2 b shown in (B) of FIG. 3to supply electric power of an appropriate voltage. In the presentembodiment, the power circuit 22 is a down-converter which converts DC6.5 V from the AC adaptor 2 a to DC 2.5 V.

The base unit voltage circuit 23 is a constant voltage source whichsupplies a stable DC voltage to the telephone section 20 and the baseunit radio section 21, and further reduces DC 2.5 V from the powercircuit 22 to DC 1.8 V.

If the voltage detection circuit 24 detects that the DC voltage from theAC adaptor 2 a is a predetermined voltage or less, the voltage detectioncircuit 24 validates a power failure detection signal to notify the baseunit control section 20 b and the switch circuit 27 that the powerfailure occurs. Further, if the voltage detection circuit 24 detectsthat the DC voltage from the AC adaptor 2 a returns to the predeterminedvoltage or more, the voltage detection circuit 24 invalidates the powerfailure detection signal to notify the base unit control section 20 band the switch circuit 27 the power return occurs.

Here, an example of the voltage detection circuit 24 will be describedin detail with reference to FIG. 5. In the present embodiment, thevoltage detection circuit 24 includes divided resistors R11 and R12which are connected to the external power connector 2 d, and atransistor Tr1 which includes a base B connected to a contact of thedivided resistor R11 and R12, a collector C connected to a power supplyVcc through a resistor R13, and an emitter E which is grounded. Thecollector C of the transistor Tr1 is connected to the switch circuit 27and is also connected to the base unit control section 20 b, so that thevoltage detection circuit 24 transmits the presence or absence of powersupply from the outside.

The base unit charging circuit 25 is a constant current source whichoutputs a constant current for charging a secondary battery BT which isinstalled in the handset 3. Here, an example of the base unit chargingcircuit 25 will be described in detail with reference to FIG. 6. Thebase unit charging circuit 25 includes transistors Tr21 and Tr22,resistors R21 and R22 and a diode D1. The transistor Tr21 includes anemitter E which is connected to the external power connector 2 d and anend of the resistor R21, a base B which is connected to the other end ofthe resistor R21 and an emitter E of the transistor Tr22, and acollector C which is connected to a base B of the transistor Tr22 and anend of the resistor R22. The transistor Tr22 includes a collector Cwhich is connected to an anode A of the diode D1. The resistor R22 hasthe other end which is grounded. A cathode K of the diode D1 whichserves as a backflow prevention circuit is connected to a terminal T11of the charging section 26 (which will be described later in detail), tothereby prevent electric current flowing as a backup current from thehandset 3 from flowing back into the base unit charging circuit 25.

The charging section 26 is a recess section 2 e which is formed on thebase unit 2 as shown in FIG. 3A, for example. The handset 3 is placed inthe recess section 2 e during charging. Two terminals T11 and T12 areinstalled on the bottom of the recess section 2 e. The terminals T11 andT12 are first terminals for supplying a charging current to thesecondary battery of the handset 3. The terminal T11 is a positiveterminal and the terminal T12 is a ground terminal which is connected toa ground of the base unit 2.

The switch circuit 27 includes a semiconductor device which has afunction of a switching element such as a transistor and an electricfield effect transistor. The switch circuit 27 becomes a cut-off stateif the power failure detection signal from the voltage detection circuit24 is invalid (non-power failure state), and becomes a connection stateif the power failure detection signal is valid (power failure state).

In the cut-off state, a line between the terminal T11 and the base unitconstant voltage circuit 23 is electrically cut off. The electricalcut-off includes a case where the charging current from the secondarybattery BT does not flow to the telephone section 20 and the base unitradio section 21, and also includes a case of a current value in whichthe telephone section 20 and the base unit radio section 21 are notoperated even though the charging current from the secondary battery BTflows thereto. Thus, the switch circuit 27 cuts off power supply to thetelephone section 20 and the base unit radio section 21 through theterminals T21 and T11 from the secondary battery BT in a case whereelectric power is supplied from the external power supply of thecordless telephone set 1.

On the other hand, in the connection state, the line between theterminal T11 and the base unit constant voltage circuit 23 iselectrically connected. The electrical connection includes a case wherethe charging current from the secondary battery BT flows to thetelephone section 20 and the base unit radio section 21, and alsoincludes any current value as long as the current value does not exceedan electric value allowed by the telephone section 20 and the base unitradio section 21 and can operate the telephone section 20 and the baseunit radio section 21. Thus, the switch circuit 27 performs power supplyto the telephone section 20 and the base unit radio section 21 throughthe terminals T21 and T11 from the secondary battery BT in a case wherepower supply from the external power supply (for example, electric powerfrom the socket) of the cordless telephone set 1 is stopped.

Here, an example of the switch circuit 27 will be described in detailwith reference to FIG. 7. The switch circuit 27 includes transistorsTr31 and Tr32, and a resistor R32. The transistor Tr31 includes a base Bwhich is connected to the voltage detection circuit 24 through aresistor R31, a collector C which is connected to an end of the resistorR32, and an emitter E which is grounded. The transistor Tr32 includes abase B which is connected to the other end of the resistor R32, anemitter E which is connected to the terminal T11, and a collector Cwhich is connected to the base unit constant voltage circuit 23.

The capacitor 28 shown in FIG. 2 is a condenser which is charged by thecharging current from the power circuit 22 or the backup current fromthe handset 3. In the present embodiment, an electric double layercondenser of a large capacity is used as the capacitor 28.

The speaker 29 is a speaker with an amplifier for activating sound fornotification of an incoming call, which is output from the base unitcontrol section 20 b.

Next, the handset 3 will be described. The handset 3 includes terminalsT21 and T22, a handset charging circuit 30, a handset constant voltagecircuit 31, a secondary battery BT, a handset constant current circuit32, a handset control section 33 which is an example of a second controlsection, a speaker section 34, a receiver section 35, a microphonesection 36, a keypad section 37, a display section 38, a handset radiosection 39 which is an example of a second radio communicating section,and a current monitoring section 40.

As shown in FIG. 4, the terminals T21 and T22 which are installed in abase end section of the handset 3 are second terminals, which are incontact with the terminal T11 and T12 when the handset 3 is placed onthe charging section 26 of the base unit 2, for input of chargingcurrent. The terminals T21 and T22 of the handset 3 are installed on thebottom section of the handset 3 as shown in FIG. 4, so as to be incontact with the terminals T11 and T12 when the bottom section of thehandset 3 is placed on the recess section 2 e of the base unit 2 shownin FIG. 3A which is the charging section 26.

The handset charging circuit 30 receives the charging current suppliedfrom the base unit 2 or the charger 4 and supplies the charging currentto the secondary battery BT. For example, as shown in FIG. 8, thehandset charging circuit 30 may have the same circuit configuration (seeFIG. 6) as that of the base unit charging circuit 25. The handsetcharging circuit 30 shown in FIG. 8 includes transistors Tr41 and Tr42,resistors R41 and R42, and a diode D2. The transistor Tr41 includes anemitter E which is connected to the terminal T21 and an end of thetransistor R41, a base B which is connected to the other end of theresistor R41 and an emitter E of the transistor Tr42, and a collector Cwhich is connected to a base B of the transistor Tr42 and an end of theresistor Tr42. The transistor Tr42 includes a collector C which isconnected to an anode A of the diode D2. The resistor R42 includes theother end thereof which is grounded. A cathode K of the diode D2 whichserves as a backflow prevention circuit is connected to the secondarybattery BT, to thereby prevent the backup current from the secondarybattery BT from flowing back into the handset charging circuit 30.

The handset constant voltage circuit 31 is a constant voltage sourcewhich supplies a stable DC voltage to the handset control section 33,the keypad section 37, the display section 38, and the handset radiosection 39, and reduces DC 2.5 V from the power circuit 22 of the baseunit 2 to DC 1.8 V.

The handset constant current circuit 32 supplies the backup current fromthe secondary current BT to the base unit 2 through the terminals T21and T11 when power supply from the outside is stopped. Here, an exampleof the handset constant current circuit 32 will be described withreference to FIG. 9.

The handset constant current circuit 32 shown in FIG. 9 may have thesame configuration (see FIG. 6) as that of the base unit chargingcircuit 25. The handset constant current circuit 32 includes transistorsTr51 and Tr52, resistors R51 and R52, and a diode D3. The transistorTr51 includes an emitter E which is connected to the secondary batteryBT and an end of the transistor R51, a base B which is connected to theother end of the resistor R51 and an emitter E of the transistor Tr52,and a collector C which is connected to a base B of the transistor Tr52and an end of the resistor R52. The transistor Tr52 includes a collectorC which is connected to an anode A of the diode D3. The resistor R52includes the other end thereof which is grounded. A cathode K of thediode D3 which serves as a backflow prevention circuit is connected tothe terminal T21, to thereby prevent the charging current from the baseunit 2 from flowing back into the handset constant current circuit 32.

The handset control section 33 shown in FIG. 2 overall controls theentire handset 3. The handset control section 33 transmits and receivesan audio digital signal to and from the base unit 2 through the handsetradio section 39 or performs control according to the power failureinformation from the base unit 2. Further, the handset control section33 notifies a received signal strength indicator (RSSI) of a radiosignal from the base unit 2 measured by the handset radio section 39 tothe base unit 2.

As shown in FIG. 4, the speaker section 34 is installed on a rear sideof the handset 3, and amplifies an audio signal from the handset controlsection 33 by the amplifier for audio output during a hand-free call.The receiver 35 is a speaker which is installed in a tip end section ofthe handset 3 for audio reproduction. The microphone section 36 receivesaudio and outputs the audio to the handset control section 33 as anaudio signal. The keypad section 37 is an operation key group whichincludes numeric keys, symbol keys and functional keys. Each key of thekeypad section 37 includes an LED which is formed of a lighttransmissive resin and functions as a backlight inside thereof Thedisplay section 38 is an LCD which performs a display on the basis ofdisplay data in the handset control section 33.

The handset radio section 39 performs modulation and demodulation forcommunication with the base unit 2 through TDMA, and transmits andreceives an audio digital signal to and from the base unit 2 through anantenna 39 a. Further, the handset radio section 39 measures thereceived signal strength of a radio signal from the base unit 2, andnotifies the result to the handset control section 33 as radio waveinformation.

The current monitoring section 40 monitors whether the handset constantcurrent circuit 32 allows electric current to flow to the base unit 2from the secondary battery BT, and notifies an electric conduction stateto the handset control section 33.

A usage state and an operation of the cordless telephone set 1 asconfigured above according to the embodiment of the invention will bedescribed.

Firstly, a normal usage state in a non-power failure state will bedescribed. When power supply is possible from the outside, DC 6.5 V issupplied from the AC adaptor 2 a. If electric power is supplied from theexternal power connector 2 d, a voltage drop occurs at the contact ofthe divided resistors R11 and R12 due to the split ratio, in the voltagedetection circuit 24 shown in FIG. 5. Accordingly, the transistor Tr1becomes in an on-state, and thus, electric current flows to the emitterE from the collector C. Thus, an output (power failure detection signal)of the collector C becomes an L level indicating the non-power failurestate (invalid). The invalidity of the power failure detection signal isindicated to the switch circuit 27, and also is indicated to the handsetcontrol section 20 b.

In the switch circuit 27, as the L level indicating the invalidity ofthe power failure detection signal is transmitted to the base B of thetransistor Tr31, the transistor Tr31 becomes an off-state. Accordingly,the transistor Tr32 also becomes in an off-state.

DC 6.5 V is supplied to the power circuit 22 from the AC adaptor 2 a. Inthe power circuit 22, DC 6.5 V is converted into 2.5 V. Then, 2.5 V isconverted to 1.8 V which is suitable for operation of the telephonesection 20 and the handset radio section 21 by the handset constantvoltage circuit 23. Further, the electric current from the AC adaptor 2a is supplied to the base unit charging circuit 25, and is supplied tothe handset 3 through the terminal T11.

In the handset 3, the electric current is supplied to the handsetcharging circuit 30 from the terminal T21 which is in contact with theterminal T11 installed in the charging section 26 of the base unit 2.Here, since the diode D3 is installed in the handset constant currentcircuit 32 and electric potentials of the terminals T11 and T21 arehigher than an electric potential on the side of the secondary batteryBT, there is no sneak current from the handset constant current circuit32.

The electric current from the handset charging circuit 30 serves as thecharging current for charging the secondary battery BT. Further, as theelectric current from the handset charging circuit 30 is supplied to thehandset constant voltage circuit 31, electric power with a suitablevoltage is supplied to the handset control section 33 and the handsetradio section 39 by the handset constant voltage circuit 31.

In this way, electric power is supplied to each section from theexternal power supply in the non-power failure state, in the cordlesstelephone set 1.

Next, an operation of the base unit 2 in the power failure state will bedescribed with reference to FIG. 10. It is assumed that the handset 3 xis placed on the charging section 26 in the base unit 2.

In the non-power failure state, since electric power is supplied fromthe AC adaptor 2 a which is the external power supply, the base unit 2performs communication with the handset 3 x, or performs a normalprocess of waiting for an incoming call through the telephone line, forexample (step S110).

If the power failure occurs, since the voltage from the AC adaptor 2 adrops, the electric potential of the contact between the dividedresistors R11 and R12 drops in the voltage detection circuit 24 of thebase unit 2. Thus, since the transistor Tr1 is in an off-state, theelectric potential of the collector C is pulled-up up to the powervoltage. Accordingly, the output (power failure detection signal) of thecollector C becomes an H level indicating the power failure state(valid). The validity of the power failure detection signal is indicatedto the switch circuit 27, and also is indicated to the handset controlsection 20 b. Further, electric current does not flow to the handset 3 xfrom the base unit charging circuit 25 due to the voltage drop from theAC adaptor 2 a. Step S110 is repeated when the power failure does notoccur (step S120).

As the H level of the power failure detection signal is output from thevoltage detection circuit 24, the transistor Tr31 becomes an on-state inthe switch circuit 27. According to the on-state of the transistor Tr31,electric current flows between the collector C and the emitter E of thetransistor Tr31 through the resistor R32 from the base B of thetransistor Tr32. As the transistor Tr32 becomes an on-state due to thevoltage drop of the resistor R31 which occurs by this electric current,the terminal T11 and the base unit constant voltage circuit 23 becomesan electric conduction state. According to the electric conduction ofthe switch circuit 27, the electric current output to the base unit 2from the secondary battery BT of the handset 3 x through the handsetconstant voltage circuit 32 as a backup current flows in the terminalsT21 and T11 and flows in the switch circuit 27, to then be supplied tothe base unit constant voltage circuit 23. Thus, the telephone section20 and the base unit radio section 21 of the base unit 2 can perform anormal operation regardless of the power failure.

Further, in the handset 3 x, as electric current from the base unitcharging circuit 25 is not supplied, charging of the secondary batteryBT is stopped, but electric power from the second battery BT is suppliedto the handset control section 33 and the handset radio section 39 ofthe handset 3 x. As electric power is supplied to the secondary batteryBT, the handset control section 33 and the handset radio section 39 canmaintain the operations (step S130).

If the base unit control section 20 b recognizes the power failure bythe power failure detection signal from the voltage detection circuit24, the base unit control section 20 b transmits a notificationindicating power supply stop to the entire handset 3 x as power failureinformation (step S140).

Further, the base unit control section 20 b switches a mode to atransmission power-saving mode in which transmission power of a radiosignal in the base unit radio section 21 is adjusted on the basis of theradio wave information from the handset 3 x (step S150). In thisadjustment, the received signal strength when the radio signaltransmitted from the base unit 2 is received by the handset 3 isdetected, information about the strength is frequently transmitted tothe base unit 2, and the base unit 2 sets transmission power on thebasis of the radio wave information from the handset 3 x.

Here, the control of the transmission power of the base unitcorresponding to the reception level in the handset will be described.FIG. 16 shows an example of the relationship between the receptionlevel, the transmission power of the base unit and the communicationerror rate in the handset, in which the transverse axis represents thereception level (RSSI: received signal strength indicator) in thehandset, and the longitudinal axis represents the transmission power ofthe base unit 2. If the transmission power of the base unit 2 islowered, the reception level in the handset 3 x is lowered, and if thereception level in the handset 3 x is lowered, a communication erroreasily occurs. In FIG. 16, an error-free point is the minimum level inwhich no communication error occurs in the handset 3 x, and it ispreferable to set the transmission power of the base unit 2 to theerror-free point or higher in order to prevent the communication errorfrom occurring. In the present embodiment, the transmission power of thebase unit 2 is controlled so that the reception level may not be lessthan the error-free point and electric power becomes the minimumpossible power (limited transmission power). As the handset 3 x and thebase unit 2 are close to each other or a communication environment isgood, the limited transmission power of the base unit 2 becomes small.

The base unit 2 receives the radio wave information which is frequentlytransmitted from the handset 3 x, and compares the received signalstrength with the reception level of the error-free point shown in FIG.16. The base unit 2 increases and decreases the transmission power basedon the comparison result. For example, in the transmission power-savingmode (power failure state), if the handset 3 x is separated from thebase unit 2, the reception level in the handset 3 x from the base unit 2is lowered. If the reception level is less than the error-free point,the base unit 2 increases the transmission power by one stage. Forexample, the transmission power of the base unit 2 is a sensitivitypoint (point in which communication error rate is 10⁻⁴) shown in FIG.16, the transmission power is increased by 10 dB from the sensitivitypoint, and thus, it is possible to return the reception level in thehandset 3 x to the error-free point. If the handset 3 x is furtherseparated from the base unit 2, the base unit 2 further increases thetransmission power. Before the communication error occurs, it ispreferable that the base unit 2 increases the transmission power by onestage. Contrarily, if the handset 3 x is close to the base unit 2, thereception level in the handset 3 x is increased from the base unit 2. Ifthe received signal strength in the handset 3 x is more than theerror-free point, a process of lowering the transmission power by onestate is performed.

For example, if the mode is switched to the transmission power-savingmode when the handset 3 x is being placed on the base unit 2, since theradio wave information from the handset 3 x initially indicates a strongvalue, the process of lowering the transmission power until the receivedsignal strength in the handset 3 x reaches the error-free point iscontinuously performed in the base unit 2, and the base unit 2considerably lowers the transmission power (see FIG. 15). Further, in acase where the handset 3 x is separated from the base unit 2, since theradio wave information from the handset 3 x becomes a weak value, theprocess of increasing the transmission power is performed so that thereceived signal intensity maintains a value higher than the error-freepoint. In this way, the communication in which no communication erroroccurs is realized with a necessary minimum transmission power (stepS150).

The presence or absence of the power failure detection signal is checkedin the next step S160, and in a case where the power failure detectionsignal from the voltage detection circuit 24 is valid, the base unit 2performs a power failure operation process (step S170). The powerfailure operation process in the base unit 2 cuts off electric power ofthe amplifier of the speaker section 29, decreases the clock frequencyof the processor used in the base unit control section 20 b, or extendsan interval at which the base unit radio section 21 monitors an emptyslot or an empty channel, thereby reducing electric power consumed bythe base unit 2.

If the power state is recovered from the power failure, the powerfailure detection signal from the voltage detection circuit 24 becomesinvalid. As the invalidity of the power failure detection signal isindicated to the base unit control section 20 b, the base unit controlsection 20 b detects the non-power failure state. According to therecovery from the power failure state, electric power is supplied to thepower circuit 22 and the base unit charging circuit 25 from the ACadaptor 2 a, and the charging current is supplied to the handset 3 xfrom the base unit charging circuit 25. Further, as the power failuredetection signal is invalid, the switch circuit 27 cuts off the electricconduction, and thus, electric current from the handset 3 x does notflow (step S180). Further, the base unit control section 20 b transmitsa notification indicating power return to the handset 3 x (step S190).In this way, the base unit 2 returns to the normal process of step S110.

Next, an operation of the handset 3 x which is placed on the chargingsection 26 of the base unit 2 will be described with reference to FIG.11.

Since electric power is supplied from the base unit 2 in the non-powerfailure state, the handset 3 x performs the normal operation (stepS210).

If power failure occurs, since the base unit 2 transmits thenotification indicating power supply stop in step S140 (see FIG. 10),the handset control section 33 of the handset 3 x receives thenotification indicating power supply stop, to thereby detect the powerfailure state. If the power state is not the power failure state, theprocedure returns to step S210 (step S220).

If the handset control section 33 detects the power failure state, thedisplay section 38 displays a message such as “External power is cutoff”, that is, a state where power supply to the base unit 2 from theexternal power supply is stopped (step S230). FIG. 17 shows an exampleof a state where this message is displayed. Further, in a case where thepower failure state is detected and the backup current flows into thebase unit 2 from the constant current circuit 32 of the handset 3 x, thedisplay section 38 displays a message such as “External power is cutoff. Please do not remove the handset” is displayed, and may display amessage for encouraging a user to maintain the state of being placed inthe charging section 26, that is, a message indicating prohibition ofthe handset 3 x from being removed from the handset 2. FIG. 18 shows anexample of a state where this message is displayed.

Next, in step S240, it is determined whether the handset 3 x receivesthe notification indicating power return from the base unit 2. In a casewhere the notification indicating power return from the base unit 2 isnot transmitted, since the power state is the power failure state, thepower failure operation is performed (step S250).

Since the power failure operation process of the handset 3 x reduces thebrightness of the backlight of the keypad section 37 or the displaysection 38 or extends the interval at which the handset radio section 39monitors the empty slot or the empty channel, electric power consumed bythe handset 3 x is reduced.

The handset control section 33 receives a radio signal from the baseunit 2, and measures the received signal strength indicator (RSSI) ofthe radio signal by the handset radio section 39. Further, the handsetcontrol section 33 transmits the received power strength measured by thehandset radio section 39 to the base unit 2 as radio wave information.The base unit 2 performs adjustment of the transmission power in stepS150 by the radio wave information.

If the base unit 2 receives the notification indicating power returntransmitted in step S190 through the handset radio section 39 (stepS240, Yes), a message such as “External power is restored” is displayedon the display section 38 for a predetermined time (step S260). Further,the procedure returns to step S210, and then the normal process isperformed. FIG. 19 shows an example of a state where this message isdisplayed.

Next, when one handset 3 x is placed on the charging section 26 of thebase unit 2, an operation of the other handset 3 y which is separatedfrom the base unit 2 will be described with reference to FIG. 12.

If the secondary battery BT of the handset 3 y is sufficiently charged,the handset 3 y is supplied with electric power from the secondarybattery BT, to thereby perform the normal process (step S310).

If a power failure occurs, since the base unit 2 transmits thenotification indicating power supply stop in step S140 (see FIG. 10),the handset control section 33 of the handset 3 y receives thenotification indicating power supply stop, and detects the power failurestate. If the power state is not the power failure state, the procedurereturns to step S310 (step S320).

As the handset control section 33 of the handset 3 y detects the powerfailure state, a power failure occurrence message such as “Externalpower is cut off” is displayed on the display section 38 (step S330).

It is determined in step S340 whether the notification indicating powerreturn from the base unit 2 is received by the handset control section33 (step 340). In a case where the notification indicating power returnis not received, since the power state is the power failure state, thepower failure operation process is performed (step S350).

In the power failure operation process of the handset 3 y, as thebrightness of the backlight of the keypad 37 or the display section 38is reduced or the interval at which the handset radio section 39monitors the interval of the empty slot and the empty channel isextended, electric power consumed by the handset 3 y is reduced.

Further, the handset control section 33 of the handset 3 y receives aradio signal from the base unit 2, and measures the received signalstrength of the radio signal by the handset radio section 39. Further,the handset control section 33 transmits the received signal strengthmeasured by the handset radio section 39 to the base unit 2 as radiowave information. The base unit 2 performs adjustment of thetransmission power in step S150 by the radio wave information.

If the handset control section 33 receives the notification indicatingpower return transmitted by the base unit 2 in step S190 through thehandset radio section 39, the handset 3 y displays a message such as“External power is restored” on the display section 38 for apredetermined time (step S360). Further, the procedure returns to stepS310, and then, the normal process is performed.

When communicating with the handset 3 y, the base unit control section20 b of the base unit 2 may receive power supply from the secondarybattery BT of the handset 3 x placed on the base unit 2, and may performthe communication with a predetermined voltage output by the secondarybattery BT.

Next, an operation of the handset (handset 3 x) in a case where anyhandset 3 is not placed on the charging section 26 of the base unit 2will be described with reference to FIGS. 13 and 14.

The handset 3 x is supplied with electric power from the chargedsecondary battery BT, thereby executing the normal process (step S410).

If a power failure occurs, the handset 3 x can detect the power failurestate by the notification indicating power supply stop transmitted fromthe base unit 2. In the present embodiment, since the capacitor 28 isinstalled in the base unit 2, the base unit 2 can operate for a while byelectric power supplied from the capacitor 28 in spite of the powerfailure state. Accordingly, as the base unit radio section 21 operateseven though power supply from the outside is stopped, the base unit 2can transmit the notification indicating power supply stop to thehandset 3 and the handset 3 x can receive the notification (step S420).Further, in a case where the operation of the base unit radio section 21of the base unit 2 is stopped, since the handset 3 x cannot communicatewith the base unit 2, the handset control section 33 of the handset 3 xrecognizes that the communication is not possible, to thereby detect thepower failure state. If the power failure state is detected in stepS410, the procedure goes to step S430, and if the power failure state isnot detected, the procedure returns to step S410 to continue the normalprocess.

The handset control section 33 of the handset 3 x displays a messagesuch as “External power is cut off. You cannot communicate with the baseunit. Please switch the mode to the power failure mode.” for encouraginga user to switch the handset 3 x to the power failure mode on thedisplay section 38, under the conditions that the power failure state isdetected, and that the handset 3 x is separated from the base unit 2 andthe backup current does not flow into the base unit 2 from the handsetconstant current circuit 32 (step S430). FIG. 20 shows an example of astate where this message is displayed.

A user views the message displayed in the display section 38 andoperates the keypad section 37 in which a functional key of setting thepower failure mode is installed. The handset control section 33determines whether the functional key of setting the power failure modeis operated. If the functional key is not operated, the procedure goesto step S430 and waits for the operation (step S440).

The handset control section 33 which is in the power failure mode as thefunctional key of setting the power failure mode is operated displays amessage such as “Please set the handset on the base unit” forencouraging a user to mount the handset 3 x on the charging section 26of the base unit 2 on the display section 38 (step S450). FIG. 21 showsan example of a state where this message is displayed.

The handset control section 33 determines whether the handset 3 x isplaced on the charging section 26 of the base unit 2 according to thepresence or absence of the backup current to the base unit 2 from thehandset constant current circuit 32 (step S460).

If it is detected that the backup current to the base unit 2 in thehandset 3 x is present, it can be determined that the handset 3 x isplaced on the charging section 26 of the base unit 2, and then theprocedure goes to step S230 (FIG. 11).

In a case where it is determined that the handset 3 x is not placed onthe charging section 26 of the base unit 2, the power failure operationprocess is performed (step S470).

In the power failure operation process, in a similar way to the stepS250 (see FIG. 11), as the brightness of the backlight of the keypadsection 37 and the display section 38 is reduced or the interval atwhich the handset radio section 39 monitors the empty slot or the emptychannel is extended, electric power consumed by the handset 3 x isreduced.

As shown in FIG. 14, subsequent to the process in step S470, the handsetcontrol section 33 determines whether a dial request is present. Thedial request means that a user operates the keypad section 37 to inputan origin telephone number. If there is no dial request, the proceduregoes to step S460 (step S480).

If there is a dial request, the handset control section 33 receives aninput of the dial (origin telephone number). The handset control section33 stores the received origin telephone number in a storing section inthe handset control section 33 (step S490).

The handset control section 33 displays a message such as “Please setthe handset on the base unit” of encouraging a user to mount the handset3 x on the charging section 26 of the base unit 2 on the display section38 (step S500). FIG. 22 shows an example of a state where this messageis displayed.

As the backup current flows to the base unit 2 from the handset constantcurrent circuit 32, the handset control section 33 determines whetherthe handset 3 x is placed on the charging section 26 of the base unit 2(step S510). If the handset 3 x is not placed on the charging section 26of the base unit 2, the procedure goes to step S500.

In a case where it is detected that the handset 3 x is placed on thecharging section 26 of the base unit 2 by the backup current, thehandset control section 33 requests the base unit 2 to make an outgoingcall to the origin telephone number stored in the storing sectionthrough the handset radio section 39. The base unit 2 makes the outgoingcall through the line interface section 20 a on the basis of the origintelephone number notified by the base unit control section 20 baccording to the request from the handset 3 x (step S520).

The handset 3 x in the power failure mode receives a user's voicethrough the microphone section 36 and outputs the other party's voicethrough the speaker section 34, thereby transitioning to the hand-freecall state (step S530).

In this way, when power supply to the base unit 2 from the outside isstopped due to the power failure or breakdown of the AC adaptor 2 a, thebase unit 2 switches the mode to the transmission power-saving mode orreduces the transmission power of the radio signal to the handset 3 x upto the level where no communication error occurs, according to the radiowave information indicating the received signal strength of the radiosignal from the base unit 2 transmitted from the handset 3 x. Forexample, as shown in FIG. 15, the transmission power is in a high outputstate in the normal operation, but in the case of the handset 3 x whichis placed on the base unit 2, it is possible to set the transmissionpower to the minimum by reducing the transmission output after the powerfailure occurs. Further, if the handset 3 x is separated from the baseunit 2, it is possible to set the transmission power to such a degreethat no communication error occurs. Further, in a case where the handset3 x is close to the base unit 2 (but is not placed on the base unit 2),it is possible to set the transmission power to be higher than the casewhere the handset 3 x is placed on the base unit 2 or to be lower thanthe case where the handset 3 x is separated from the base unit 2.Accordingly, according to the distance between the base unit 2 and thehandset 3 x, it is possible to suppress power consumption on the side ofthe base unit 2 as much as possible and to continue a voice call for arelatively long time.

Further, since the handset control section 33 of the handset 3 canautomatically make a phone call through the hand-free call functionaccording to detection of the power failure state through thenotification indicating power supply stop from the base unit 2, it ispossible to start the call in a state where the handset 3 x is placed onthe base unit 2 without any operation.

Further, if the power failure state is detected, the base unit 2 cannotify the power abnormality to each user who uses the handset 3 bytransmitting the notification indicating power supply stop to eachhandset 3, or can encourage the user to mount the handset 3 on thecharging section 26 of the base unit 2.

Since the capacitor 28 which supplies electric power to the base unitconstant voltage circuit 23 is installed in the base unit 2, it ispossible to transmit the notification indicating power supply stop tothe handset 3 on the basis of the notification from the voltagedetection circuit 24 even in the power failure state. Further, eventhough the handset 3 is in the state of being separated from thecharging section 26 of the base unit 2 during a call, since the baseunit 2 can be operated by electric power supplied from the capacitor 28,it is possible to maintain the call in a state where the handset 3 isseparated from the base unit 2 for a while, without cease of the callfrom the handset 3.

In the case of the base unit 2 in a state where electric power cannot besupplied from the outside as the handset 3 x is placed on the chargingsection 26 of the base unit 2, since electric power is supplied by thesecond battery BT from the handset 3 x, the handset 3 y which isseparated from the base unit 2 can make a call.

In the base unit 2 of the cordless telephone set 1 according to thepresent embodiment, the power circuit 22 and the base unit constantvoltage circuit 23 are separately configured, but may be combined as apower circuit. Further, the power circuit may be configured to furtherinclude the base unit charging circuit 25.

Further, when the power failure occurs during a call as the handset 3 xis separated from the base unit 2, if there is no problem even thoughthe call is ceased, it is possible to omit the capacitor 28.

In the above-described embodiment, the communicating section and thecontrol section are separately configured, but may be integrallyconfigured. For example, the communication section and the controlsection may be configured as a semiconductor integrated circuit whichhas the functions of the communication section and the control section.

In the above-described embodiment, when the handset 3 of the cordlesstelephone set 1 is placed on the base unit 2, the terminals T21 and T22of the handset 3 and the terminals T11 and T12 of the base unit 2 arephysically in contact with each other, but the invention is not limitedthereto as long as electric power can be supplied to the base unit 2from the secondary battery BT of the handset 3 in the cordless telephoneset 1. This may include an electrical connection or a magneticconnection. For example, a transformer may be installed instead of theterminals T21 and T22 on the side of the handset 3, a transformer may beinstalled instead of the terminals T11 and T12 on the side of the baseunit 2, and the transformer of the handset 3 and the transformer of thebase unit 2 may be magnetically combined, to thereby supply electricpower.

The invention has been described in detail with reference to thespecific embodiments, but it will be apparent to those skilled in theart that a variety of modifications or revisions may be made in a rangewithout departing from the spirit or scope of the invention.

This application is based on Japanese Patent Application No. 2009-183009filed on Aug. 6, 2009, the contents of which are incorporated herein byreference.

INDUSTRIAL APPLICABILITY

The invention can lengthen the time for a call by suppressing powerconsumption during a power failure, and is thus suitable for a cordlesstelephone set including a handset which is supplied with electric powerfrom a secondary battery and a base unit which is supplied with electricpower from a power line.

REFERENCE SIGNS LIST

-   1 CORDLESS TELEPHONE SET-   2 BASE UNIT-   2 a AC ADAPTOR-   2 b RECESS SECTION-   2 c DIAL-UP CONNECTOR-   2 d EXTERNAL POWER CONNECTOR-   2 e RECESS SECTION-   20 TELEPHONE SECTION-   20 a LINE INTERFACE SECTION-   20 b BASE UNIT CONTROL SECTION-   21 BASE UNIT RADIO SECTION-   21 a ANTENNA-   22 POWER CIRCUIT-   23 BASE UNIT CONSTANT VOLTAGE CIRCUIT-   24 VOLTAGE DETECTION CIRCUIT-   25 BASE UNIT CHARGING CIRCUIT-   26 CHARGING SECTION-   27 SWITCH CIRCUIT-   28 CAPACITOR-   29 SPEAKER SECTION-   T11, T12 TERMINAL-   3, 3 x, 3 y HANDSET-   30 HANDSET CHARGING CIRCUIT-   31 HANDSET CONSTANT VOLTAGE CIRCUIT-   32 HANDSET CONSTANT CURRENT CIRCUIT-   33 HANDSET CONTROL SECTION-   34 SPEAKER SECTION-   35 RECEIVER SECTION-   36 MICROPHONE SECTION-   37 KEYPAD SECTION-   38 DISPLAY SECTION-   39 HANDSET RADIO SECTION-   39 a ANTENNA-   40 CURRENT MONITORING SECTION-   BT SECONDARY BATTERY-   T21, T22 TERMINAL-   4 CHARGER-   4 a AC ADAPTOR

1. A cordless telephone set which performs radio communication between ahandset and a base unit which is provided with an external power supply,wherein the base unit includes: a detection unit which detects theexternal power supply, wherein the handset includes: a battery whichprovides a power supply to the base unit in accordance with the externalpower supply detected by the detection unit; and a display section whichdisplays information regarding at least one of the power supply and theexternal power supply.
 2. The cordless telephone set according to claim1, wherein the base unit further includes: a first radio communicatingsection which performs modulation and demodulation with the externalpower supply; and a first control section which controls transmissionpower of the first radio communicating section, wherein the handsetfurther includes: a second radio communicating section which performsmodulation and demodulation with a voltage output from the battery,wherein the first radio communicating section receives, from thehandset, radio wave information which indicates a received signalstrength of a radio signal which is received by the second radiocommunicating section from the first radio communicating section, andthe first control section reduces the transmission power of the firstradio communicating section in response to the received signal strengthindicated by the radio wave information on the basis of the externalpower detected by the detection unit.
 3. The cordless telephone setaccording to claim 2, wherein the first control section reduces thetransmission power of the first radio communicating section down to alevel at which no communication error occurs.
 4. The cordless telephoneset according to claim 1, wherein the handset further includes a secondcontrol section which starts up a hand-free call function if the handsetis placed on the base unit.
 5. The cordless telephone set according toclaim 2, wherein the first radio communicating section transmitspredetermined data to the handset when the detection unit has detectedthat a voltage from the external power supply does not exceed apredetermined value, and the display section displays a state wherepower supply to the base unit from the external power supply is cut offwhen the second radio communicating section receives the predetermineddata.
 6. The cordless telephone set according to claim 1, wherein thebase unit further includes a capacitor which receives electricity withthe voltage from the external power supply or the voltage from thebattery.
 7. The cordless telephone set according to claim 2, wherein thecordless telephone set includes a plurality of handsets, and when anyone of the plurality of handsets is placed on the base unit, the firstradio communicating section communicates with a different handset whichis not placed on the base unit with a predetermined voltage output fromthe battery.
 8. The cordless telephone set according to claim 2, thefirst radio communicating section transmits predetermined information tothe handset on the basis of the external power supply detected by thedetection unit, and the display section performs a display forencouraging a user to place the handset on the base unit when thedisplay section has received the predetermined information by the secondradio communicating section.
 9. The cordless telephone set according toclaim 2, further comprising: a switch circuit which cuts off powersupply to the first radio communicating section from the secondarybattery of the handset placed on the base unit when the detection unithas detected that a voltage from the external power supply exceeds apredetermined value and allows power supply to the first radiocommunicating section from the secondary battery of the handset placedon the base unit when the detection unit has detected that the voltagefrom the external power supply does not exceed the predetermined value.10. The cordless telephone set according to claim 9, wherein the firstradio communicating section transmits predetermined information to thehandset when the detection unit has detected that the voltage from theexternal power supply does not exceed the predetermined value, and thedisplay section performs a display for preventing the handset placed onthe base unit from being removed from the base unit when the displaysection has received the predetermined information by the second radiocommunicating section and the switch circuit allows power supply to thefirst radio communicating section from the secondary battery.
 11. Thecordless telephone set according to claim 1, wherein the display sectiondisplays information indicating that the external power is unavailable.12. The cordless telephone set according to claim 1, wherein the displaysection displays information for keeping the handset placed on the baseunit.
 13. The cordless telephone set according to claim 1, wherein thedisplay section displays information that the external power supply isavailable if the detection unit detects that a voltage from the externalpower supply exceeds a predetermined value.
 14. A power supplying methodwhich is performed between a handset and a base unit which is providedwith an external power supply, the method comprising: detecting theexternal power supply; providing a power supply from the handset to thebase unit in accordance with the external power supply; and displayinginformation regarding at least one of the power supply and the externalpower supply.
 15. A cordless handset which performs radio communicationwith a base unit, the handset comprising: a battery which provides apower supply to the base unit in accordance with a status of an externalpower supply to the base unit, a display section which displaysinformation regarding at least one of the power supply and the externalpower supply.
 16. A power supplying method implemented by a handsetwhich performs radio communication with a base unit, the methodcomprising: providing a power supply from the handset to the base unitin accordance with a status of an external power supply to the baseunit; and displaying information regarding at least one of the powersupply and the external power supply.
 17. A base unit which is providedwith an external power supply and performs radio communication with acordless handset, the base unit comprising: a detection unit whichdetects the external power supply; and a switch circuit which receives astatus of the external power supply from the detection unit, andconnects to a battery of the handset to receive a backup power supplyfrom the battery of the handset in accordance with the status of theexternal power supply.
 18. A power receiving method implemented by abase unit which performs radio communication with a cordless handset,the method comprising: detecting the external power supply; andconnecting the base unit to a battery of the handset to receive a backuppower supply from the battery of the handset in accordance with a statusof the external power supply.